text: Enzyme responsive gated materials are another class of gated delivery systems. In these scenarios, enzymes can trigger release of the gates from the scaffolds in drug delivery systems. The mechanism for this type of gate is that certain linkages are used that can be hydrolyzed by select enzymes. The two most popular choices are protease and hyaluronidase. An advantage of using enzyme responsive triggers is that there is a large amount of substrate specificity, and the enzymes are able to trigger their target with high selectivity, even under mild conditions. Another advantage of this system is that enzymes are found throughout the entire body and work on almost all biological processes so the delivery system could potentially be activated in any part of the body during many points within a singular process. One study done by the Martinez-Manez lab in Valencia, Spain aimed to generate MSNs linked to poly-l-glutamic acid (PGA) gates through peptide bonds. The trigger for this system was the presence of a lysosomal proteolytic enzyme (protease), in this case, pronase. The researchers found that in the absence of pronase, the system was only able to release less than 20% of its cargo in 24 hours, however, in the presence of pronase, there was a 90% release of cargo within 5 hours.